This invention is directed to the field of overpressure relief devices for vacuum systems, and, more particularly, to overpressure devices that rupture when a vacuum system is pressurized.
A typical vacuum system easily tolerates a higher pressure on the outside than the inside (negative pressure), but is susceptible to damage when there is a higher pressure on the inside than the outside (positive pressure). In many systems, even a small amount of positive pressure may cause damage to expensive equipment, such as manipulators, connected to a vacuum system. Accordingly, where possible vacuum systems often incorporate pressure relief devices to vent the system if the internal pressure exceeds a predetermined amount, which amount may be as little as 1 psig.
Overpressure for vacuum systems is most likely to occur when systems are connected to an external source of gas used for known reasons such as purging, chemical vapor deposition, or sputter cleaning. Some internal gas sources, such as cryopumps and uranium hydride beds, can also release enough gas to overpressurize a vacuum system.
An effective pressure relief device for a vacuum system must meet several criteria. Most importantly, it must be able to resist the negative pressure the system is designed to undergo during normal vacuum operations. On the other hand, without any operator interaction, it must also reliably vent the system when the positive pressure exceeds a predetermined value. The device must also be impervious to corrosive chemicals which may be used within the system.
U. S. Pat. No. 4,512,419 of DeGood et al. discloses a rupture disc, one conventional pressure relief device. The design principle employed in a rupture disc to ensure predicable bursting pressures is simply a tension failure of the single metal membrane of the disc, the overpressure escaping through the ruptured disc. The design problem for a rupture disc is making it strong enough to resist negative pressure, and weak enough to fail at a predetermined positive pressure.
The reverse acting rupture disk is another overpressure device where a diaphragm has a bulging surface pulled by the negative pressure towards the interior of the vacuum system. In the event of overpressure, the bulging surface is pushed outwardly, where it is pierced by a knife blade, causing the diaphragm to rupture.
A problem with rupture discs is that the size of the disc varies inversely with the rupture pressure. For example, one commercially available disc has a diameter of 24" to provide 4 PSIG overpressure protection. The large size and cost of such a disc limits its usefulness in many applications.